netdev - [PATCH v2 bpf-next 01/14] libbpf: ensure ELF symbols table is found before further ELF processing

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Message-ID: <20200901015003.2871861-2-andriin@fb.com>
Date:   Mon, 31 Aug 2020 18:49:50 -0700
From:   Andrii Nakryiko <andriin@...com>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>
CC:     <andrii.nakryiko@...il.com>, <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>
Subject: [PATCH v2 bpf-next 01/14] libbpf: ensure ELF symbols table is found before further ELF processing

libbpf ELF parsing logic might need symbols available before ELF parsing is
completed, so we need to make sure that symbols table section is found in
a separate pass before all the subsequent sections are processed.

Signed-off-by: Andrii Nakryiko <andriin@...com>
---
 tools/lib/bpf/libbpf.c | 40 +++++++++++++++++++++++++++++-----------
 1 file changed, 29 insertions(+), 11 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index b688aadf09c5..ac56d4db6d3e 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2720,14 +2720,38 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 	Elf *elf = obj->efile.elf;
 	Elf_Data *btf_ext_data = NULL;
 	Elf_Data *btf_data = NULL;
-	Elf_Scn *scn = NULL;
 	int idx = 0, err = 0;
+	const char *name;
+	Elf_Data *data;
+	Elf_Scn *scn;
+	GElf_Shdr sh;
 
+	/* a bunch of ELF parsing functionality depends on processing symbols,
+	 * so do the first pass and find the symbol table
+	 */
+	scn = NULL;
 	while ((scn = elf_nextscn(elf, scn)) != NULL) {
-		const char *name;
-		GElf_Shdr sh;
-		Elf_Data *data;
+		if (elf_sec_hdr(obj, scn, &sh))
+			return -LIBBPF_ERRNO__FORMAT;
+
+		if (sh.sh_type == SHT_SYMTAB) {
+			if (obj->efile.symbols) {
+				pr_warn("elf: multiple symbol tables in %s\n", obj->path);
+				return -LIBBPF_ERRNO__FORMAT;
+			}
 
+			data = elf_sec_data(obj, scn);
+			if (!data)
+				return -LIBBPF_ERRNO__FORMAT;
+
+			obj->efile.symbols = data;
+			obj->efile.symbols_shndx = elf_ndxscn(scn);
+			obj->efile.strtabidx = sh.sh_link;
+		}
+	}
+
+	scn = NULL;
+	while ((scn = elf_nextscn(elf, scn)) != NULL) {
 		idx++;
 
 		if (elf_sec_hdr(obj, scn, &sh))
@@ -2766,13 +2790,7 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 		} else if (strcmp(name, BTF_EXT_ELF_SEC) == 0) {
 			btf_ext_data = data;
 		} else if (sh.sh_type == SHT_SYMTAB) {
-			if (obj->efile.symbols) {
-				pr_warn("elf: multiple symbol tables in %s\n", obj->path);
-				return -LIBBPF_ERRNO__FORMAT;
-			}
-			obj->efile.symbols = data;
-			obj->efile.symbols_shndx = idx;
-			obj->efile.strtabidx = sh.sh_link;
+			/* already processed during the first pass above */
 		} else if (sh.sh_type == SHT_PROGBITS && data->d_size > 0) {
 			if (sh.sh_flags & SHF_EXECINSTR) {
 				if (strcmp(name, ".text") == 0)
-- 
2.24.1